As demand for existing wireless services grows and new services continue to emerge, there will be an increasing need for antennas on vehicles. Existing antenna technology usually involves monopole or whip antennas that protrude from the surface of the vehicle. These antennas are typically narrow band. Thus, to address a wide variety of communication systems, it is necessary to have numerous such antennas positioned at various locations on the vehicle. Furthermore, as data rates continue to increase, especially with 3G, Bluetooth, direct satellite radio broadcast, and wireless Internet services, the need for antenna diversity will increase. This means that each individual vehicle will require multiple antennas each operating in different frequency bands, and/or with different polarizations and/or at different elevations relative to the horizon. Since vehicle design is often dictated by styling, the presence of numerous protruding antennas will not be easily tolerated. Furthermore, the installation of multiple antennas is costly.
The most basic prior art antenna is the simple whip monopole that is used for FM radio reception and cellular phones. The antenna has a nearly omnidirectional radiation pattern, producing a null only towards the sky. The primary disadvantage of the monopole antenna is that it protrudes from the exterior of the vehicle as an unsightly vertical wire with a height of roughly one quarter wavelength. The monopole is also typically narrowband with a bandwidth of roughly 10%. In order to access multiple wireless services operating on multiple frequencies, multiple monopole antenna would thus be required. Furthermore, if antenna diversity is used to provide directional sensitivity, the number of required antennas is even greater. A logical alternative might be to use a single broadband antenna that could cover all frequency bands of interest. Examples of broadband antennas include spiral antennas, flared notch antennas and log periodic antennas. However, with all of these types of antennas and with broadband antennas in general, the presence of the metallic ground plane is not tolerated. However, if a part of the metal vehicle body is removed and replaced with a dielectric, such a broadband antenna could be integrated into this dielectric and would function over a broad bandwidth. This concept is shown in FIG. 1. A significant drawback of this approach is that it allows the interior of the vehicle to receive just as much antenna radiation as the exterior. With increasing questions over the effects of electromagnetic radiation, this design may be undesirable.
Antennas exist which can function well in the presence of the metallic ground plane, such as patch antennas and various types of traveling wave antennas. These antenna all tend to excite surface currents in a surrounding ground plane. Such surface currents can tend to cause interaction between the individual antennas and can also cause radiation to occur at discontinuities or at edges of the ground plane. This problem is shown in FIG. 2.
Thin antennas exist, such as patch antennas; however, they typically exhibit a narrow bandwidth and do not provide flexibility in the shape of the radiation pattern and/or their sensitivity pattern. Conversely, broadband antennas exist, but they generally are not thin and/or they cannot tolerate the presence of a nearby metal ground plane. One possible solution is to eliminate the metallic ground plane by removing a portion of the vehicle frame or body and replacing it with a sheet of dielectric. A thin broadband antenna can then be mounted on the dielectric sheet to provide access to many wireless services. The problem with this solution is that the elimination of the ground plane allows radiation inside the vehicle interior. Furthermore, with many services sharing the same antenna, interference between devices within the vehicle is increased.
The prior art includes the following:    1) D. Sievenpiper and E. Yablonovitch, “Circuit and Method for Eliminating Surface Currents on Metals” U.S. provisional patent application, serial No. 60/079953, filed on Mar. 30, 1998 by UCLA and corresponding PCT application PCT/US99/06884, published as WO99/50929 on Oct. 7, 1999, the disclosures of which are hereby incorporated herein by reference. These applications disclose a Hi-Z surface.    2) U.S. Pat. No. 4,821,040 entitled “Circular Microstrip Vehicular RF Antenna”, assigned to Ball Corporation of Muncie, Ind. This patent describes an antenna consisting of a circular slot radiator that may be mounted within the roof of a vehicle.    3) U.S. Pat. No. 6,091,367 entitled “Light-weight Flat Antenna Device Tolerant of Temperature Variation”, by Kabashima, Shigenori; Ozaki, Tsuyoshi; Takahashi, Toru; Konishi, Yoshihiko; and Ohtsuka, Masataka. This patent describes an array of multiple conventional patch antennas.    4) U.S. Pat. No. 6,037,912 entitled “Low Profile Bi-Directional Antenna”, by DeMarre, Allen G. This patent describes a low-profile antenna system for mounting on the exterior of a vehicle.    5) U.S. Pat. No. 5,850,198 entitled “Flat Antenna with Low Overall Height”, by Lindenmeier, Heinz; Hopf, Jochen; and Reiter, Leopold. This patent describes an antenna for accessing multiple frequency bands for multiple RF services by providing multiple resonant regions that act as separate antennas.    6) U.S. Pat. No. 5,818,394 entitled “Flat antenna”, by Aminzadeh, Mehran; Burkert, Manfred; Daginnus, Michael; and Chen, Shun-Ping. This patent describes an antenna mounted below the windshield of a vehicle, resulting in a low-profile design which is hidden from view.    7) U.S. Pat. No. 5,682,168 entitled “Hidden Vehicle Antennas”, by James, Jesse C.; and Blackmon, Jr., James B. This patent describes a way of mounting antennas in motor vehicles.    8) U.S. Pat. No. 5,177,493 entitled “Antenna Device For Movable Body”, by Kawamura, Katsuaki. This patent describes a method of mounting an antenna on a vehicle.    9) U.S. Pat. No. 4,760,402 entitled “Antenna System Incorporated in the Air Spoiler of an Automobile”, by Mizuno, Hiroshi; Sakurai, Takashi; and Shibata, Yoshihisa. This patent describes a way of hiding an antenna in the air spoiler of a vehicle.
Still there is a need for a single antenna unit that combines antennas for various services, and can be installed in a vehicle simply, preferably in one operation. This antenna unit should be thin and should contain a ground plane that can be integrated with or made to cooperate with the metal exterior of the vehicle so as to avoid vehicle interior radiation. Furthermore, this antenna unit should allow access to multiple wireless services which means it should operate in multiple frequency bands. For the reasons described above, it should contain several separate antennas with each antenna operating at a single band. These individual antennas should be isolated from one another and should also not allow radiation to leak into the interior of the vehicle, such as through surface currents. In order to cooperate to vehicle styling considerations, this antenna unit should also be covered by a smooth surface that can be painted to match the color of the vehicle on which it is installed. To enable low-cost installation, it should have a single connector that supplies DC power and provides an RF interface to each antenna.
Related art includes the following patent applications which are assigned to assignee of the present invention:    1) D. F. Sievenpiper, J. H. Schaffner, “A Textured Surface Having High Electromagnetic Impedance in Multiple Frequency Bands”, U.S. patent application Ser. No. 09/713,119 filed Nov. 14, 2000, the disclosure of which is hereby incorporated herein by reference. A Hi-Z surface with multiple band capability is disclosed by this US patent application.    2) D. F. Sievenpiper; J. H. Schaffner; H. P. Shu; G. Tangonan, “A Method of Providing Increased Low-Angle Radiation in an Antenna” U.S. patent application Ser. No. 09/905,796 filed on Jul. 13, 2001, the disclosure of which is hereby incorporated herein by reference. A crossed slot antenna able to receive vertically and circularly polarized RF signals is disclosed by this application.    3) D. F. Sievenpiper; J. Piluiski; J. H. Schaffner; T. Y. Hsu “Molded High Impedance Surface and A Method of Making Same” U.S. patent application Ser. No. 09/905,794 filed on Jul. 13, 2001, the disclosure of which is hereby incorporated herein by reference. An inexpensive and flexible Hi-Z surface is disclosed by this application.    4) D. Sievenpiper, H. P. Hsu, G. Tangonan, “Planar Antenna with Switched Beam Diversity for Interference Reduction in Mobile Environment”, U.S. patent application Ser. No. 09/525,831 filed Mar. 15, 2000, the disclosure of which is hereby incorporated herein by reference.    5) D. Sievenpiper; A. Schmitz; J. Schaffner; G. Tangonan; T. Y. Hsu; R. Y. Loo; R. S. Miles, “A Low-cost HDMI-D Packaging Technique for Integrating an Efficient Reconfigurable Antenna Array with RF MEMS Switches and a High Impedance Surface” U.S. patent application Ser. No. 09/906,035 filed on Jul. 13, 2001, the disclosure of which is hereby incorporated herein by reference.